1. Field of the Invention
The present invention generally relates to a focusing module for use with a divergent light source, and more particularly, the invention relates to a focusing module for a laser diode which can be adjusted to focus the output of such a diode prior to being affixed in a permanent focused position.
2. Description of the Prior Art
Various means for generating a laser beam are known in the prior art and may include a light source such as a laser tube or a semiconductor laser diode of a continuous wave or pulse type. A laser diode is much smaller in size and lighter in weight relative to a laser tube and therefore is particularly desirable for applications where size and weight requirements are to be minimized. One such application is a small, light weight hand held bar code scanner such as that described in U.S. Pat. No. 4,496,831. However, laser diodes emit a beam of light which diverges as the light moves outwardly along a centralized axis of emission. Typical laser diode light diverges 10.degree.-20.degree. in one plane through the axis of emission and 40.degree.-50.degree. in a second plane through the axis of emission normal to the first plane. The axis of emission is defined as the axis around which the light diverges symmetrically.
Such angular divergence is unacceptable for bar code scanning applications, which require a beam spot, preferably rectangular in cross-section having a relatively well defined edge. To achieve such a beam spot with the light of a laser diode, the divergence must be reversed using a lens system, thereby converging or focusing the light's intensity to a well defined spot at the point at which the bar code is to be read.
In the prior art, such lens systems for converging and focusing emitted laser diode light were embodied in one of a series of co-axial cylindrical members, one of which received the laser diode and was affixed thereto. The member remote from the diode was closed, having a circular opening therein which was coaxial with the emission axis of the diode and the central axis of the member supporting the diode. This opening allowed emission of the now converging beam, which was focused by the lens at a position located a distance from the emitter of the diode along the above axis. The lens was mounted in the member remote from the diode. Motion of the lens in the axial direction was prevented by providing a force such as from a compressed spring which extended the interior length of the two members and was coaxial with the central axis the cylindrical members, where one end engaged the lens and its opposite end rested on the washer-like face which defined the emission aperture of the received laser diode.
Movement in the axial direction was permitted through a construction whereby the members were provided with threaded portions on the interior or exterior surface which contacted the other member and interfaced with the corresponding threaded portion on the surface of the other member. These threads allowed one member to be rotated with respect to the other, changing their relative positions along the central axis, and consequently moving the lens system along the central axis with respect to the laser diode. Therefore, by rotating the cylindrical members with respect to one another, the focus of the laser light was adjusted.
The lens system of the prior art suffered from a number of disadvantages, with one such disadvantage being the cost of the focusing module. Due to the threads required on the members to focus the laser light and permit axial movement between the cylindrical members, the cylindrical members had to be precisely machined, which is relatively expensive with respect to other metal working techniques and which consequently increased the cost of the end product in which the focusing module was used.
Another disadvantage in the prior art was that the lens was seated directly on a washer-like surface of one of the cylindrical members, which necessitated that this surface have a forward taper to match the portion of the profile of the lens surface with which it contacted, in order to align and maintain a coaxial orientation of the lens axis with the central axis of the members. If the washer-like surface were normal to the annular portions, the seat would be made through contact with the innermost ledge of the washer, making the lens susceptible to tilting and chipping. On the other hand, a tapered surface provided a ring of contact by the seat, avoiding the above, but this taper required that the washer-like surface be machined off a right angle portion of the surface, resulting in a forward annular portion of relatively small radius. Therefore, the opening in the front of the cylindrical member was smaller than desirable, and served to mask the light which passed through the lens, decreasing the intensity of the focused spot. The intensity of the beam passing through the opening is critical in applications such as bar code scanning, wherein reliability is a function of the sharpness of the reflected light as well as the intensity. To compensate for this loss of intensity, the prior art focusing module had to be precisely aligned with respect to the emission axis, thereby achieving a more discretely focused spot at the point of scanning; however, this precision required the dimensional tolerances of the members of the focusing module to be very small, which could only be achieved by machining the critical dimensions at a high cost. Dimensions of the threaded interfaces were critical, and had to prevent even marginal axial and radial movement between the cylindrical housing members which would result in misalignment of the lens and decrease focusing of the emitted light, causing an asymmetrical intensity pattern at the point of focus.
A further disadvantage laid in that the opening of the focusing module for emission of laser light had to be circular by this design, since any other shape would not maintain its orientation with respect to the laser diode about the central axis after the cylindrical members were rotated in the focusing operation. Therefore not only was the focusing module opening circular, it had to be of small diameter, and had the further effect of limiting the focused intensity for the scanning operation.
A still further disadvantage was encountered during the focusing process, when one cylindrical housing member was rotated with respect to the other. Since the lens made direct contact with the rotating cylindrical member and the stationary spring, the lens rotated with respect to the cylindrical housing member, the positioning spring, or both. This relative rotation tended to misalign the central axis of the lens, and led to scratching or scoring of the lens as it moved relative to the housing member and spring.